1. Field of the Invention
The invention relates to the field of casting processes and apparatus for lenses fabricated from liquid plastics.
2. Prior Art
In recent years considerable attention in the opthalmic industry has been given to plastic lenses for eyeglasses. This interest has been prompted in part by governmental regulations on the shatter-resistance of lenses and by the popularity of larger eyeglass lenses. It is costly to meet new shatter-resistance standards or form these larger lenses with glass, in addition to the fact that larger glass lenses are relatively heavy and cannot be tinted.
Methods have been developed for fabricating these plastic lenses by casting them from liquid plastic. A transparent plastic such as a thermosetting allyl diglycol carbonate, known commercially as "CR-39" and distributed by Pittsburgh Plate Glass Company, is commonly used. This liquid monomer is solidified after mixing with a catalyst such as isopropyl percarbonate or isopropyl peroxide. The solidification of this liquid plastic is characterized by substantial shrinkage. Numerous techniques have been developed to permit the fabrication of optical surfaces despite this shrinkage.
The currently most widely used method for forming the "CR-39" lenses employs two glass mold halves spaced apart by an annular gasket. The liquid plastic is injected through the gasket into a cavity defined between the glass mold halves. A clamp is employed to apply pressure to these mold halves in order to keep the mold halves in contact with the solidifying plastic and to prevent leaking. For a discussion of this method, see U.S. Pat. Nos. 3,070,846 and 3,136,000. Among the problems with this prior art casting method is that a plurality of different gaskets are required in order to provide the typical range of prescription lenses and moreover, the glass mold halves are relatively expensive and not always reuseable.
Another prior art method for forming these plastic lenses in which a flexible element is used to form an upper and lower cavity in a single mold is described in U.S. Pat. No. 3,211,811.
Still another prior art technique for forming lenses from a liquid plastic employing a press is described in U.S. Pat. No. 3,380,718.
As will be seen, the invented process and apparatus does not require a plurality of different gaskets. Moreover, in the presently preferred embodiment, optical surfaces are obtained employing plastic molds, without polishing the cast lenses.